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1990 yamaha 570 exciter bogs

I am having trouble with my snowmobile bogging down. It starts and runs fine for the first few minutes. Once it warms up, I have trouble with it bogging down around 4000-4500 rpm. I can feather it to get it above 5000 rpm and it flys. I've tried cleaning the carbs twice and replaced all fuel lines and spark plugs and new gas. I've checked compression when cold and measures 130-135 per cylinder. I also blocked all ports and pressurized the engine to 8-10lbs. and noticed a small leak(foaming soap) around the crank seal and bigger leak around the exhaust gaskets. A parts salesmen is convinced I need to replace the crank seals. Does the symptoms sound right for a crank seal leaking? Any advise on changing the seals? This is my first time tearing an engine down.

8-10lbs sound like quite a bit for and engine pressure test. I think it only needs to hold 3-5 psi. You may have overpressurized your crank case. Back to your problem, sounds like your belt is wore or your secondary is not downshifting like it should. Where does the belt sit in the sheaves? It should be flush or just a hair above the the sheave when not moving.

I think your right. I removed the belt gaurd and noticed a chunk missing from my belt. Also, the belt is sitting about 1/8 to 3/16 of inch below the major diameter of the secondary clutch. I am not for sure what you are talking about downshifting. What would cause it to not down shift? Thanks for the help stinger.

When the secondary opens and the belt goes down the sheaves it is upshifting, When the motor slows down the secondary comes back together and belt rides back up the sheaves to the top. This is downshifting. A broken spring, wore out cam sliders, bad bushings etc can prevent the sheaves from coming all the way back together and not downshifting all the way. Thus its like trying to start off in 3rd gear. A wore down belt will do the same thing as it does not come all the way back to the top due to it being thinner. The engine then bogs a bit until it gets enough RPM's to pull that "gear"

Well, I have pulled the secondary clutch apart and thoroughly cleaned it and reassembled with a new belt. I have been out riding a couple of times and it runs much, much, better now.
I may have to take a look at my primary clutch though, now it seems like the clutch wants to start to engage around 2700 but I don't really start to move until 3500. I am thinking, maybe some weak springs?

Well, I did replace the fuel pump when I cleaned the secondary clutch. Since then, I have been out riding 3 times and and it has been running pretty good. The only complaint I would have is this machine seems like it goes through alot of gas. About 4-5 mpg the best I could figure. Is this normal? I am going to be out riding again tomorrow. We will see if I have any problems.

Well, I have pulled the secondary clutch apart and thoroughly cleaned it and reassembled with a new belt. I have been out riding a couple of times and it runs much, much, better now.
I may have to take a look at my primary clutch though, now it seems like the clutch wants to start to engage around 2700 but I don't really start to move until 3500. I am thinking, maybe some weak springs?

I'm no expert on these sleds, but I think normally a new belt will bring the RPM's down a little bit, but that seems a little much. Are you using an OEM Yammie belt?

When the secondary opens and the belt goes down the sheaves it is upshifting, When the motor slows down the secondary comes back together and belt rides back up the sheaves to the top. This is downshifting. A broken spring, wore out cam sliders, bad bushings etc can prevent the sheaves from coming all the way back together and not downshifting all the way. Thus its like trying to start off in 3rd gear. A wore down belt will do the same thing as it does not come all the way back to the top due to it being thinner. The engine then bogs a bit until it gets enough RPM's to pull that "gear"

I have a similar problem, most of the time everythings good, when running almost full throttle in deep powder, all of a sudden the rpms will raise way high while the sled abrublty slows down to stop, after the rpms return to idle, can go again.

at first i thought it was excessive snow entering and making belt slip, but not sure now, sometimes i could open hood and some snow would be there and others none. i am clueless,
1992 exciter 570 l/c has new comet primary, and the slides for the secondary were replaced,

could it be the position of the helix(spring tension for the secondary)?

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